Process for the chemical treatment of sand for glassmaking



rLnll Patented Jan. 22, 1952 PROCESS FOR THE CHEMICAL TREATMENT OF SAND FOR GLASSMAKING Herbert Daymond Segrove, Thornton Heath, England, assignor to Standard Brick 8: Sand Company Limited, Holmethorpe, Redhill, England No Drawing. Application December 14, 1949, Se-

rial No. 133,000. In Great Britain December 10 Claims.

In my application Serial No. 61,129, now Patent No. 2,556,005, I have disclosed the treatment of sand having a ferruginous coating by wetting the grains of sand with a caustic alkali solution, heating the wetted sand to substantial dryness and thereafter treating the sand with a mineral acid. At present there is a considerable shortage of alkali in Great Britain and the necessary supplies to maintain a large plant operating this process cannot readily be obtained.

It has been shown that various silicates, clays and sand which. contain iron as an impurity can be purified by removal of iron when treated with hydrosulphurous acid H2820; and mineral acid at normal temperatures. The use of sulphuric acid was mentioned in this connection. The use of aldehyde and ketone sulphoxylic acids has been suggested in place of hydrosulphurous acid.

For the production of sand suitable for glass making it has also been proposed in British Specification No. 555,241 to use a solution of titanous sulphate and hydrofluoric acid. This is stated to form a fluotitanate. Instead of using hydrofluoric acid, sodium silicofluoride may be used. Titanous sulphate is a useful reagent but it is very expensive especially when large quantities of material are to be treated.

A quite substantial reduction in the iron content of sand can be obtained by treatment with sulphuric-acid and sodium hydrosulphite but the results are not as good as those obtainable by the process of my aforesaid application and it has been observed that there is a considerable tendency for irregularity in the results which it has not been possible to account for.

'It has now been found that good and consistent results can be obtained by agitating. sand with a dilute aqueous solution of hydrofluoric acid containing hydrosulphurous acid or an at least sparingly soluble hydrosulphite.

Accordingly the present invention provides a process for the removal of a ferruginous coating from sand which comprises agitating the sand with a dilute aqueous solution of hydrofluoric acid containing hydrosulphurous acid or an at least sparingly soluble hydrosulphite. Preferably the hydrosulphite is a water-soluble alkali metal hydrosulphite.

I have found that very useful results can be obtained at ordinary temperatures using a very dilute aqueous solution of hydrofluoric acid containing an alkali metal hydrosulphite dissolved therein provided that it is agitated. Ordinary stirring is not sufficient, substantial agitation is necessary and vigorous agitation is preferred. The concentrations of hydrofluoric acid employed may be so low that they are not injurious to the health of the workers under the prevailing conditions. Under comparable conditions hydrofluoric acid alone effects substantially no reduction in the ferruginous content of the treated sand and the same is true of sodium hydrosulphite. Carrying out tests under the same con ditions with hydrofluoric acid and sulphuric acid in amounts which gave equivalent hydrogen ion concentrations and using the same amount of sodium hydrosulphite a much greater reduction in iron content was obtained with the treating solu- I tion containing hydrofluoric acid.

Only a small reduction in iron content was observed when sand was treated with an aqueous solution of sodium hydrosulphite and it appears that the noted results can only be obtained by the simultaneous use of hydrofluoric acid and hydrosulphurous acid or an at least sparingly soluble hydrosulphite and agitating. Other sulphur containing compounds such as sodium sul- -phide, sodium metabisulphite and sodium bisulphite which are known to behave as reducing agents have little or no action upon ferruginous sand when applied in dilute solution.

In carrying out the process of the invention the hydrofluoric acid may be 0.01% to 1.0% by weight of commercial hydrofluoric acid of approximately strength (i. e. 0.00'7-0.'7% by weight of HF) together with 0.02 to 0.2% by weight of sodium hydrosulphite or its molar equivalent of other soluble hydrosulphite (i. e. 0.0011 to 0.0115 moi per kilogram of reagent).

Whilst the time of treatment is dependent to a substantial extent upon the degree of dilution of the applied rea ents with the concentrations indicated periods of treatment of about 5 minutes, or at most 10 minutes are suflicient. It is preferred to keep the concentration of the hydrosulphite as low as possible as otherwise sulphurous gases may be evolved which are a nuisance. At the concentrations indicated above this does not occur.

The following table shows the results obtained when a number of substances were agitated with sand that had been subjected to cleaning followed by froth flotation to remove iron-contain- F810; content Treating solution (a) before (b) after treatment treatment 1. 0.08% hydrofluoric acid 0.054 0.052 2. 0.02% sodium hydrosulphite 0.052 0.050 3. 0.087 hydrofluoric acid and 0.02%

sodium hydrosulphite with agitation 0. 055 0. 035 4. 0.22% sulphuric acid and 0.02%

sodium hydrosulphite with agi-- tation 0. 056 0. 046 5 0.08 7 hydrofluoric acid and 0.12%

so ium hydrosulphite with agitation 0. 052 0. 026 6. 0.227 sulphuric acid and 0.12%

sodium hydrosulphite with agitation 0. 056 0. 044 7. 0.087hydrofiuoric acid and 0.02%

so lum iormaldehyde sulphxy-' late with agitation 0.050 8. 0.087 hydrofluoric acid and 0.02%

sodium hydrosulphite (without agitation). 0.058 I 0. 058

The weights of hydrofluoric acid given in the above table are based upon 100% acid HF.

In the above tests hydrofluoric and sulphuric acids were used in solutions having substantially the same pH value and it will be seen that hydrofluoric acid gives much better results. In other tests using more strongly acid solutions deferrugination did not appear to be substantially improved. Hydrosulphurous acid is unstable at pH values below about 3.

It is also notable that no deferrugination was secured with low concentrations of hydrofluoric acid and sodium hydrosulphite in the absence of agitation. This suggests a combination of mechanical and chemical action.

The following examples illustrate the manner in which the invention may be carried into effect: a

Example 1 Sand which contained 0.135% of FezOs after rough washing with water still contained 0.064% of F8203 after vigorous washing. It was then agitated with an aqueous solution containing 0.2% of 40% hydrofluoric acid and 0.02% of sodium hydrosulphite for minutes. This reduced the F820: content to 0.047% and after froth flotation to remove iron-containing mineral particles in the froth and leave a silicious gangue, the F8203 content was further reduced to 0.027%.

Example 2 In another similar trial using similar starting material the FezQa content was reduced to 0.026%.

Examples 3 i0 6 In four further trials different samples of sand were analysed after rough washing and again after treatment with hydrofluoric acid (0.2% by weight of 40% acid) sodium hydrosulphite (0.02% by weight) solution and froth flotation. The following results were obtained:

4 It will be seen that the combination of the hydrofluoric-hydrosulphurous acid treatment with a froth flotation step, which may be applied before or after the acid treatment, results in 5 substantially complete deferrugination and gives a product satisfactory for glass-making. The froth flotation is preferably one in which the mineral particles are removed with the froth leaving a residue of sand which is then further washed.

Examples 7 to 12 In the following examples the time of treatment was standardised at 5 minutes and a numher of samples of sand having a widely differing iron content were submitted to treatment.

F9203 content after treatment outlined Example No 7 0 Treatment:

After Washing 0.080 0.052 0.003 0.042 0. 050 0.005 Aiterfiotation 0. 052 0.042 0.050 0.020 0. 051 0.050 After treatment (with 0.02%

NagSgO; and

and with agitation) 0.022 0. 020 0.022 0.018 0.028 0.027

Examples 13 to 15 In the following three examples samples were taken from batches of sand after various times of agitation with solutions of hydrofluoric acid containing sodium hydrosulphite with the fol lowin results:

rao. content Before Reagents treat- Time of treatment in mins.

ment

No.13: W Percent 3I5Z%2; ,%f;}0055 0. 044 0.041 0. 039 0.025 0. 035

In Example 14 the treatment was apparently substantially complete after 2 minutes and the variation in the figures from that time illustrates the factor of experimental error in sampling the slurries. A reagent having the concentration of that used in Example 13 is regarded as a typical satisfactory one in that the deferrugination was apparently complete'in less than 5 minutes whilst the loss of hydrosulphite by oxidation is less than with a more concentrated reagent and the loss by hydrolysis is less than in a more strongly acid solution. In Example 15 the reaction was slower and probably not complete in 10 minutes. The longer period of agitation involved leads to a greater loss of hydrosulphite by oxidation.

Sufllcient agitation appears to be necessary to prevent the slurry of sand from settling. Sand is a relatively coarse and rapid settling material and the vessel in which the agitation is carried out should be provided with an impeller which can be operated at a sufficient speed to keep the sand in the vessel in suspension. Using a laboratory flotation machine having a cruciform impeller set in a circular base, speeds of 1500-2000 R. P. M. have been found satisfactory. With a larger machine lower speeds will probably 'be suflicient since it is believed that it is the peripheral speed of the impeller and the diameter of the vessel which are the operative factors.

There are certain advantages in this method oi treatment. No heat is required. The solutions applied are extremely dilute which makes for economy in the consumption of chemicals and also means that the hydrofluoric acid is not present in amounts injurious to the workers. The time of contact of the reagent with the sand is also short.

I claim.

1. A process for the removal of a ierruginons coating from sand which comprises agitating sand with a dilute aqueous solution of hydrofluoric acid containing a substance selected from the group consisting of hydrosulphurous acid and an alkali metal hydrosulphite.

2. A process for the removal of a ferruginous coating from sand which comprises maintaining sand in suspension in a dilute aqueous solution of hydrofluoric acid containing a substance selected from the group consisting of hydrosul phurous acid and an alkali metal hydrosulp'nite.

3. A process for the deierrugination or" sand which comprises maintaining sand in suspension in a dilute aqueous solution of hydrofluoric acid containing a substance selected from the group consisting of hydrosulphurous acid and alkali metal hydrosulphites and thereafter subjecting the sand to a froth flotation step to separate floatable iron-containing particles therefrom.

4. A process for the removal of a ferruginous coating from sand which comprises maintaining sand in suspension in a dilute aqueous solution of hydrofluoric acid containing a low concentration of sodium hydrosulphite.

5. A process for the removal of a ferruginous 6 coating from sand which comprises maintaining sand in suspension in a dilute aqueous solution containing 0.007-0.7% by weight of hydrofluoric acid and 0.0011-0.015 moi of water soluole alkali metal hydrosulphite.

A process for the removal of a ferruginous I coating from sand which comprises maintaining sand in suspension in a dilute aqueous solution containing (LOW-0.7% by weight of hydrofluoric acid and 0.02-0.2% by weight of sodium hydrosulphite.

'7. The process of claim 6 in which the sand is suspended in said solution for not more than 10 minutes.

8. A process for the deferrugination of sand which comprises maintaining sand in suspension in a dilute aqueous solution containing (LOW-0.7% by weight of hydrofluoric acid and (LUZ-0.20% by weight of sodium hydrosulphite and thereafter subjecting the treated sand to a froth flotation step to separate floata-ble ironcontaining particles therefrom.

9. A process for the deferrugination of sand which comprises subjecting sand to a froth flotation step to separate floatable iron-containing particles therefrom and thereafter treating said sand in suspension in a dilute aqueous solution containing OHM-0.7% by weight of hydrofluoric acid and 0.02-0.20% by weight of sodium hydrosulphite.

10. A process for the deferrugination of sand which comprises subjecting sand to a froth flotation step to separate fioatable iron-containing particles therefrom and thereafter treating said sand in suspension in a dilute aqueous solution of hydrofluoric acid containing a substance selected from the group consisting of hydrosulphurous acid and an alkali metal hydrosulphite.

HERBERT DAYMOND SEGROVE.

No references cited. 

1. A PROCESS FOR THE REMOVAL OF A FERRUGINOUS COATING FROM SAND WHICH COMPRISES AGITATING SAND WITH A DILUTE AQUEOUS SOLUTION OF HYDROFLUORIC ACID CONTAINING A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF HYDROSULPHUROUS ACID AND AN ALKALI METAL HYDROSULPHITE. 